It may be an existing topic but couldn't find it.
I tried updating IX4-200d to cloud version though USB method and post reboot NAS box has gone crazy, it is now rebooting constantly after displaying IOMEGA logo.

Tried downgrading firmware using USB but nothing seems to be working, please help !!!

Thanks for prompt reply, I have gone through this post but it couldn't help, below is status of IX4-200d

1. It shows IOmega logo , shows blue screen and then reboots.
2. seems it is not reaching to state where it can see USB connected and reboots prior to this stage.
3. I have got another working IX4-200d non cloud, I took out all disks from non working box, pulled a disk from working box and plugged in non working IX4, still situation is same, it reboots constantly.
4. Booting it without HDD is also yielding same results.

On my ix2-200d the kernel and initrd are stored in flash. I guess that is also the case on yours. In that case you have flashed the cloud version of the kernel and initrd, which apparently is not compatible.
The easiest way I know to solve that is to connect to the serial port, stop the bootloader, upload the right kernel and initrd (which can be taken from your non-ce recovery stick) by serial or by tftp (the latter being *far* faster), and boot it with and empty disk, and the recovery stick inserted. The box will recover, and part of that is that it will flash the kernel and initrd from stick to the right place in flash.
Note: that serial port is TTL level. So you can't connect it directly to an rs232 port.

1. How to set IP on NAS interface which can connect to TFTP server.
2. Other end of cable has 4 pins. this NAS has 10 Pin header, not sure which pin to connect where. Looking at IX2 I have connected pins in below order

You don't have a 4 pin header? Checked both sides of the board? Bummer.

Here I wrote a little story about find the serial port. A TX can also be found using an oscilloscope, but I'm aware not everybody has one around. A piezo buzzer can also be used
It will generate 'acoustic modem' -like sound when connected between ground and TX (when there is signal, the first few seconds after power-on). Alas RX is harder to find, as it's passive.

I meant a bit higher resolution. Have you also checked the unpopulated header left of the barcode sticker?

is there anyway to reset NAND / Mother board so that it goes to USB init?

I'm afraid not. The script which is responsible for initiating the USB init is in NAND (where else could it be?) and is overwritten by the (incompatible) Cloud version. So now there is nothing left which can even read USB.